A sound level meter that uses a microphone of a sound pressure type has been used in evaluating acoustic performance in an enclosed space such as a room of a building, vehicle or airplane.
However, when sound pressure measurement takes place in the enclosed space, the presence of a standing wave significantly affects the sound pressure measured. In this case, the standing wave refers to characteristic vibrations that occur in such cases as when the dimensions of the enclosed space match the wavelength. When the standing wave occurs in the enclosed space, the following phenomenon could occur: Large sound pressure is measured at a given measurement location, while small sound pressure is measured at another measurement location.
Thus, the problem is that when acoustic performance is evaluated in an enclosed space such as a room of a building, it is impossible to accurately evaluate acoustic performance, such as performance of sound insulation of the space, by measuring sound pressure only at one point due to the effects of the standing wave. To address such a problem, what is defined in JIS or ISO is to measure sound pressure at a plurality of points in the space and to average the measured values in a bid to improve the accuracy of acoustic performance evaluation. For example, what is disclosed in Patent Document 1 is a housing acoustic performance measurement device that adopts such an ISO standard.
Those disclosed in Patent Documents 2 and 3 are used as a technique of correcting a sound field with a standing wave being taken into account. The technique disclosed in Patent Document 2 is designed to measure a standing wave in a trunk room when a car audio system outputs sound in a vehicle, cancel the standing wave with the output of a separately provided speaker to eliminate the standing wave in the trunk room, and therefore output good-quality sound in the vehicle. Similarly, the technique disclosed in Patent Document 3 is designed to measure a standing wave, cancel the standing wave at a given location with the output of a speaker and stabilize a sound pressure level.